Patent GB-1200870-A (GB)
Title: Improvements in electrical waveguide arrangements
Inventor: CRAVEN GEORGE FREDERICK
IPC: H01P1/00
Filed: 1969-06-06
Abstract: 1,200,870. Waveguide junctions and bends. STANDARD TELEPHONE & CABLES Ltd. 6 June, 1969, No. 28698/69. Heading H1W. The arms of a hybrid T-junction, a simple T-junction or a waveguide bend are realized as resonant sections operating in evanescent modes i.e. at frequencies for which the waveguides are normally dimensioned beyond out-off. The invention is based upon the fact that a length of evanescent mode waveguide presents an inductive reactance to an incident wave and that propagation of energy through the length may be realized by providing a capacitative reactance which is the conjugate of the inductive reactance. The resulting sections may be of either T or TT form depending upon whether a single capacitance midway along the section, or a pair capacitances at the ends of the section, are used. In the hybrid junction of Fig. 1, symmetrical arms 1 of length l 1 , nominally dimensioned beyond cut-off, are caused to propagate energy by suitably adjusting capacitative screws 4, 5 arranged midway along the arms. Shunt arm 2 forms part of a series resonant T-section of length l 2  which extends from the centre line of the arms 1 to the end of the arm 2, capacitance being provided by a central screw 6. Series arm 3 comprises a TT-section parallel-resonant section of length l 3  having terminal capacitances consisting respectively of a conductive screw 10 and a dielectric screw 8 extending into a dielectric window 7. It is shown in the specification that the necessary matching conditions and quadrature phase conditions between arms 2, 3 may be realized by properly adjusting the parameters of this embodiment. The loaded Q's of the resonant sections are made equal and input/output connected to the symmetrical arms. The junction behaves as an all-pass phase-equalizing network. A number of such junctions may be coupled in tandem. In a modification, one of the symmetrical arms 1, Fig. 20, is provided at a central point with a capacitive screw 5 and forms a series-resonant circuit. The other symmetrical arm 1 contains a dielectric plate 40 at one end and a screw 4 at the other; it forms a parallel-resonant circuit. The arms reflect energy from an input 2 or 3 in phase quadrature towards an output 3 or 2 and may be used to effect phase equalization of e.g. a filter. Matching of the shunt and series arms is effected by screws 6, 8, 10 and dielectric plate 7. The invention may also be applied to E-plane and H-plane T-junctions which are formed respectively by short-circuiting the shunt and the series arms of a hybrid junction e.g. as shown in Fig. 1 or Fig. 20. The evanescent-mode E-plane T-junction shown in Fig. 22 is provided with capacitive reactances in the form of dielectric plates 7, 40A, 40B. The junction comprises three TT-section evanescent-mode resonators each of half-length l/2. E-plane and H-plane evanescent-mode waveguide bends may be formed by short-circuiting one of the symmetrical arms of a T-junction. The Specification gives several examples (apart from Fig. 22) of bends and T-junctions using both screws and plates as capacitive elements.